The present invention relates to cubic boron nitride (CBN) materials and more particularly to a CBN product, which has very high toughness and high thermal stability.
Cubic boron nitride (CBN) is the second hardest material known to man after diamond. The manufacture of CBN by the high pressure/high temperature (HP/HT) process is known in the art and is typified by the process described in U.S. Pat. No. 2,947,617, a basic monocrystalline CBN case. U.S. Pat. No. 4,188,194 describes a process for making sintered polycrystalline CBN compacts which utilizes pyrolytic hexagonal boron nitride (PBN) in the absence of any catalyst. An improvement on such direct conversion process is disclosed in U.S. Pat. No. 4,289,503 wherein boric oxide is removed from the surface of the HBN powder before the conversion process. U.S. Pat. No. 5,106,792 manufactures a CBN composite mass from a mixture of different structural forms of graphitic boron nitride (CBN) in the absence of any catalyst.
A compact is a mass of abrasive particles bonded together in a self-bonded relationship (see U.S. Pat. Nos. 3,852,078 and 3,876,751); by means of a bonding medium (U.S. Pat. Nos. 3,136,615, 3,233,988, 3,743,489, 3,767,371, and 3,918,931); or by means of combinations thereof. A composite compact is a compact bonded to a substrate material, such as cemented metal carbide. U.S. Pat. No. 3,918,219 teaches the catalytic conversion of hexagonal boron nitride (HBN) to CBN in contact with a carbide mass to form a composite CBN compact. Compacts or composite compacts may be used in blanks for cutting tools, drill bits, dressing tools, and wear parts (see, for example, U.S. Pat. Nos. 3,136,615 and 3,233,988).
Polycrystalline CBN compacts often are used in machining hard ferrous alloy workpieces due to their relatively non-reactivity with ferrous workpieces. Accordingly, CBN materials often are formed into cutting, milling, and turning tools. The toughness of the CBN crystals, as measured by a standard friability test, can be a factor in grinding performance. The friability test involves ball milling a quantity of product under controlled conditions and sieving the residue to measure the breakdown of said product. The toughness index (TI) is measured at room temperature, and the thermal toughness index (TTI) is measured after the product has been fired at a high temperature. In many cases, the tougher the crystal, the longer the life of the crystal in a grinding or machining tool and, therefore, the longer the life of the tool. This leads to less tool wear and, ultimately, lower overall tool cost.
Corrigan, et al., “Direct Transition Among Allotropic Forms of Boron Nitride at High Pressures and Temperatures”, The Journal of Chemical Physics, Vol 63. No. 9, page 3812 (Nov. 1, 1975) discusses the effects of impurities (e.g., oxygen) in the HP/HT conversion of HBN to CBN (see page 3814). Dreger, et al., “Sublimation and Decomposition Studies on Boron Nitride and Aluminum Nitride”, J. Phys. Chem., 66, p. 1556 (1962) proposes heating BN to 1200°–2000° C. to evolve nitrogen gas and leave a coating of boron. Vacuum firing of isotropic HBN to remove boron oxide preparatory to metallizing is mentioned in U.S. Pat. No. 3,926,571 at col. 3. Preliminary drying of HBN is disclosed in U.S. Pat. No. 4,150,098 at col. 3. Finally, U.S. Pat. No. 4,289,503 pre-treats HBN at a temperature in the HBN thermal decomposition range by vacuum firing or heating under inert atmosphere to remove boric oxide and leave a surface coating of boron. The pre-treated HBN, then, is converted into CBN.